A known method for synthesizing sine waves is to generate harmonic frequencies from a reference clock and filter a selected harmonic frequency to obtain a desired frequency output. Another known method is to use a phase-locked loop having a digital divider in its feedback loop. Neither of these methods allows for phase continuous switching of the carrier, and both methods require extensive analog components which are subject to drift and malfunction through aging, temperature effects and the like.
A digital signal generator synthesizer is useful to avoid the above problems. A block diagram of a typical digital synthesizer known to the art is shown in FIG. 1. The defining relationship for frequency is F=(.DELTA..phi./.DELTA.T)(1/2.pi.). If .DELTA.T is the period of the digital clock, then .DELTA.T uniquely defines a frequency. It is known that frequency and phase modulation are simply obtainable from this synthesizer. Amplitude modulation, AM hereafter, is more difficult and is typically provided by a multiplier either before or after the digital-to-analog converter. If the multiplier is after the digital-to-analog converter, the structure takes the form of a common AM analog modulator. If the multiplier is inserted before the digital-to-analog converter, a complex digital multiplier is required.
Amplitude modulating after the digital-to-analog conversion is disadvantageous in that it prevents the entire generator from being in digital form. Also, the modulation index is not digitally controllable in a simple manner, and the analog multiplier degrades the carrier in terms of harmonics and spurious signal generation.
A digital multiplier approach eliminates these problems. Referring to FIG. 2, a typical arrangement for sine wave carrier amplitude modulation is shown. The modulating source may be any random wave with higher frequencies filtered to prevent aliasing. The carrier term sin (WcT) is typically derived from a ROM look-up phase-to-amplitude converter. The modulation term is 1+MRm(T), where M is the modulation index and Rm(T) is the modulating wave. This approach, although digital, presents several additional problems. The maximum carrier frequency is limited by the speed of the multiplier.
To reduce the spurious signals that result from too coarse an amplitude resolution at least 10 bits are typically needed thereby requiring a 10.times.10 type multiplier. State-of-the-art multipliers of this size are very complex. Maximum speed is about 90 nsec. per multiplication, and the addition of the multiplier and the 1+MRm(T) generator approximately doubles the complexity of a synthesizer.